Methods and Patterning Devices and Apparatuses for Measuring Focus Performance of a Lithographic Apparatus, Device Manufacturing Method

ABSTRACT

Disclosed is a method of measuring focus performance of a lithographic apparatus. The method comprises using the lithographic apparatus to print at least one focus metrology pattern on a substrate, the printed focus metrology pattern comprising at least a first periodic array of features, and using inspection radiation to measure asymmetry between opposite portions of a diffraction spectrum for the first periodic array in the printed focus metrology pattern. A measurement of focus performance is derived based at least in part on the asymmetry measured. The first periodic array comprises a repeating arrangement of a space region having no features and a pattern region having at least one first feature comprising sub-features projecting from a main body and at least one second feature; and wherein the first feature and second feature are in sufficient proximity to be effectively detected as a single feature during measurement. A patterning device comprising said first periodic array is also disclosed.

FIELD OF THE INVENTION

The present invention relates to inspection apparatus and methodsusable, for example, to perform metrology in the manufacture of devicesby lithographic techniques. The invention further relates to suchmethods for monitoring a focus parameter in a lithographic process.

BACKGROUND ART

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,including part of, one, or several dies) on a substrate (e.g., a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned.

In lithographic processes, it is desirable frequently to makemeasurements of the structures created, e.g., for process control andverification. Various tools for making such measurements are known,including scanning electron microscopes, which are often used to measurecritical dimension (CD), and specialized tools to measure overlay, theaccuracy of alignment of two layers in a device. Recently, various formsof scatterometers have been developed for use in the lithographic field.These devices direct a beam of radiation onto a target and measure oneor more properties of the scattered radiation—e.g., intensity at asingle angle of reflection as a function of wavelength; intensity at oneor more wavelengths as a function of reflected angle; or polarization asa function of reflected angle—to obtain a diffraction “spectrum” fromwhich a property of interest of the target can be determined.

Examples of known scatterometers include angle-resolved scatterometersof the type described in US2006033921A1 and US2010201963A1. The targetsused by such scatterometers are relatively large, e.g., 40 μm by 40 μm,gratings and the measurement beam generates a spot that is smaller thanthe grating (i.e., the grating is underfilled). Diffraction-basedoverlay metrology using dark-field imaging of the diffraction ordersenables measurement of overlay and other parameters on smaller targets.These targets can be smaller than the illumination spot and may besurrounded by product structures on a substrate. The intensities fromthe environment product structures can efficiently be separated from theintensities from the overlay target with the dark-field detection in theimage-plane.

Examples of dark field imaging metrology can be found in internationalpatent applications US20100328655A1 and US2011069292A1 which documentsare hereby incorporated by reference in their entirety. Furtherdevelopments of the technique have been described in published patentpublications US20110027704A, US20110043791A, US2011102753A1,US20120044470A, US20120123581A, US20130258310A, US20130271740A andWO2013178422A1. These targets can be smaller than the illumination spotand may be surrounded by product structures on a wafer. Multiplegratings can be measured in one image, using a composite grating target.The contents of all these applications are also incorporated herein byreference.

One important parameter of a lithographic process which requiresmonitoring is focus. There is a desire to integrate an ever-increasingnumber of electronic components in an IC. To realize this, it isnecessary to decrease the size of the components and therefore toincrease the resolution of the projection system, so that increasinglysmaller details, or line widths, can be projected on a target portion ofthe substrate. As the critical dimension (CD) in lithography shrinks,consistency of focus, both across a substrate and between substrates,becomes increasingly important. CD is the dimension of a feature orfeatures (such as the gate width of a transistor) for which variationswill cause undesirable variation in physical properties of the feature.

Traditionally, optimal settings were determined by “send-ahead wafers”i.e. substrates that are exposed, developed and measured in advance of aproduction run. In the send-ahead wafers, test structures were exposedin a so-called focus-energy matrix (FEM) and the best focus and energy(exposure dose) settings were determined from examination of those teststructures. More recently, focus metrology targets are included in theproduction designs, to allow continuous monitoring of focus performance.These metrology targets should permit rapid measurements of focus, toallow fast performance measurement in high-volume manufacturing.Ideally, the metrology targets should be small enough that they can beplaced among the product features without undue loss of space.

Current test structure designs and focus measuring methods have a numberof drawbacks. Known focus metrology targets require sub-resolutionfeatures and/or grating structures with large pitches. Such structuresmay contravene design rules of the users of lithographic apparatuses.Asymmetry in a grating structure can be measured effectively usinghigh-speed inspection apparatus such as a scatterometer, working atvisible radiation wavelengths. Known focus measuring techniques exploitthe fact that focus-sensitive asymmetry can be introduced intostructures printed in a resist layer by special design of the patternson a patterning device that defines the target structure. For EUVlithography, where printing is performed using radiation of a wavelengthless than 20 nm, for example 13.5 nm, the creation of sub-resolutionfeatures becomes even more difficult. For EUV lithography, resistthickness, and therefore the thickness of target structures, is smaller.This weakens the diffraction efficiency, and hence the signal strength,available for focus metrology.

For these reasons, there is a need to develop new techniques for themeasurement of focus performance in lithographic processes, particularlyin EUV lithography, but also for projection-based lithography ingeneral.

SUMMARY OF THE INVENTION

The present invention aims to provide alternative methods of measuringfocus performance. In some aspects the invention aims to provide methodsthat are adaptable to new environments, such as EUV lithography. In someaspects, the invention aims to avoid the requirement for sub-resolutionfeatures to be defined in a patterning device.

In a first aspect of the invention, the inventors have recognized thatalternative target designs can be devised, which provide focus-dependentasymmetry signals without the use of sub-resolution features.

The invention in a first aspect provides a method of measuring focusperformance of a lithographic apparatus, the method comprising: (a)obtaining measurement data relating to measured asymmetry betweenopposite portions of a diffraction spectrum for a first periodic arrayin a printed focus metrology pattern on a substrate; and (b) deriving ameasurement of focus performance based at least in part on the asymmetrycomprised within the measurement data, wherein said first periodic arraycomprises a repeating arrangement of a space region having no featuresand a pattern region having at least one first feature comprisingsub-features projecting from a main body and at least one secondfeature; and wherein the first feature and second feature are insufficient proximity to be effectively detected as a single feature whenmeasured in a measurement step.

The invention in a second aspect provides a patterning device for use ina lithographic apparatus, the patterning device comprising reflectiveand non-reflective portions to define features of one or more devicepatterns and one or more metrology patterns, the metrology patternsincluding at least one focus metrology pattern, the focus metrologypattern comprising at least a first periodic array of featurescomprising a repeating arrangement of features arranged to define aspace region having no features and a pattern region having at least onefirst feature comprising sub-features projecting from a main body and atleast one second feature; and wherein the first feature and secondfeature are in sufficient proximity to be effectively detected as asingle feature during a scatterometery based metrology action to measureasymmetry between opposite portions of a diffraction spectrum for thefirst periodic array as formed on a substrate.

The invention yet further provides a lithographic system comprising alithographic apparatus comprising:

an illumination optical system arranged to illuminate a reflectivepatterning device;

a projection optical system arranged to project an image of thepatterning device onto a substrate; and

a metrology apparatus according to the first aspect of the invention asset forth above,

wherein the lithographic apparatus is arranged to use the measurement offocus performance derived by the metrology apparatus when applying thepattern to further substrates.

The invention yet further provides computer program products for use inimplementing methods and apparatuses according to various aspects of theinvention as set forth above.

The invention yet further provides a method of manufacturing devicesusing the method according to the first aspect or the second aspect ofthe invention as set forth above.

Further features and advantages of the invention, as well as thestructure and operation of various embodiments of the invention, aredescribed in detail below with reference to the accompanying drawings.It is noted that the invention is not limited to the specificembodiments described herein. Such embodiments are presented herein forillustrative purposes only. Additional embodiments will be apparent topersons skilled in the relevant art(s) based on the teachings containedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus having a reflective patterningdevice;

FIG. 2 depicts a lithographic cell or cluster in which a lithographicapparatus and metrology apparatus can be used to perform methodsaccording to the present invention;

FIGS. 3a and 3b illustrate schematically an inspection apparatus adaptedto perform angle-resolved scatterometry and dark-field imaginginspection methods;

FIG. 4 shows a prior described example focus metrology pattern;

FIG. 5 illustrates the formation of a focus metrology target on asubstrate using a reflective patterning device in one embodiment of thepresent invention;

FIGS. 6a, 6b, 6c show schematically, examples of focus metrologypatterns for use in embodiments of the invention;

FIGS. 7a, 7b, 7c, 7d, 7e, 7f show schematically, further examples offocus metrology patterns for use in embodiments of the invention;

FIGS. 8a and 8b illustrate two complementary variants of the focusmetrology target pattern shown in FIG. 5(a);

FIG. 9 shows the formation of a composite focus metrology targetcomprising complementary variants of focus metrology patterns of thetype shown in FIG. 8(a) and (b);

FIG. 10 shows a dark-field image of the metrology focus patterns of thetarget of FIG. 9, obtained using the apparatus of FIG. 3; and

FIG. 11 is a flowchart of a method of monitoring focus according to anembodiment of the invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Before describing embodiments of the invention in detail, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

FIG. 1 schematically depicts a lithographic apparatus 100 including asource module SO according to one embodiment of the invention. Theapparatus comprises:

-   -   an illumination system (illuminator) IL configured to condition        a radiation beam B (e.g. EUV radiation).    -   a support structure (e.g. a mask table) MT constructed to        support a patterning device (e.g. a mask or a reticle) MA and        connected to a first positioner PM configured to accurately        position the patterning device;    -   a substrate table (e.g. a wafer table) WT constructed to hold a        substrate (e.g. a resist-coated wafer) W and connected to a        second positioner PW configured to accurately position the        substrate; and    -   a projection system (e.g. a reflective projection system) PS        configured to project a pattern imparted to the radiation beam B        by patterning device MA onto a target portion C (e.g. comprising        one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure MT holds the patterning device MA in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem.

The term “patterning device” should be broadly interpreted as referringto any device that can be used to impart a radiation beam with a patternin its cross-section such as to create a pattern in a target portion ofthe substrate. The pattern imparted to the radiation beam may correspondto a particular functional layer in a device being created in the targetportion, such as an integrated circuit.

In general patterning devices used in lithography may be transmissive orreflective. Examples of patterning devices include masks, programmablemirror arrays, and programmable LCD panels. Masks are well known inlithography, and include mask types such as binary, alternatingphase-shift, and attenuated phase-shift, as well as various hybrid masktypes. An example of a programmable mirror array employs a matrixarrangement of small mirrors, each of which can be individually tiltedso as to reflect an incoming radiation beam in different directions. Thetilted mirrors impart a pattern in a radiation beam which is reflectedby the mirror matrix.

The projection system, like the illumination system, may include varioustypes of optical components, such as refractive, reflective, magnetic,electromagnetic, electrostatic or other types of optical components, orany combination thereof, as appropriate for the exposure radiation beingused, or for other factors such as the use of a vacuum. It may bedesired to use a vacuum for EUV radiation since other gases may absorbtoo much radiation. A vacuum environment may therefore be provided tothe whole beam path with the aid of a vacuum wall and vacuum pumps.

As here depicted, the apparatus is of a reflective type (e.g. employinga reflective mask). The focus metrology techniques of the presentdisclosure have been developed particularly for use with reflectivepatterning devices (reticles), where illumination is not in a directionnormal to a plane of the patterning device surface, but at a slightlyoblique angle. In principle, the same techniques could apply in relationto a transmissive patterning device, if for some reason illuminationintroduced asymmetry. Conventionally, illumination of the reticle isdesigned to be symmetrical, but with reflective reticles, that is notgenerally possible.

Certain embodiments of the present disclosure exploit asymmetry in theprojection system using a reflective patterning device. Otherembodiments are applicable with any kind of projection system.

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives an extreme ultra violetradiation beam from the source module SO. Methods to produce EUV lightinclude, but are not necessarily limited to, converting a material intoa plasma state that has at least one element, e.g., xenon, lithium ortin, with one or more emission lines in the EUV range. In one suchmethod, often termed laser produced plasma (“LPP”) the required plasmacan be produced by irradiating a fuel, such as a droplet, stream orcluster of material having the required line-emitting element, with alaser beam. The source module SO may be part of an EUV radiation systemincluding a laser, not shown in FIG. 1, for providing the laser beamexciting the fuel. The resulting plasma emits output radiation, e.g.,EUV radiation, which is collected using a radiation collector, disposedin the source module. The laser and the source module may be separateentities, for example when a CO2 laser is used to provide the laser beamfor fuel excitation.

In such cases, the laser is not considered to form part of thelithographic apparatus and the radiation beam is passed from the laserto the source module with the aid of a beam delivery system comprising,for example, suitable directing mirrors and/or a beam expander. In othercases the source may be an integral part of the source module, forexample when the source is a discharge produced plasma EUV generator,often termed as a DPP source.

The illuminator IL may comprise an adjuster for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as a-outer anda-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as facetted field and pupilmirror devices. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the support structure (e.g., mask table) MT, and ispatterned by the patterning device. After being reflected from thepatterning device (e.g. mask) MA, the radiation beam B passes throughthe projection system PS, which focuses the beam onto a target portion Cof the substrate W. With the aid of the second positioner PW andposition sensor PS2 (e.g. an interferometric device, linear encoder orcapacitive sensor), the substrate table WT can be moved accurately, e.g.so as to position different target portions C in the path of theradiation beam B. Similarly, the first positioner PM and anotherposition sensor PS1 can be used to accurately position the patterningdevice (e.g. mask) MA with respect to the path of the radiation beam B.Patterning device (e.g. mask) MA and substrate W may be aligned usingmask alignment marks M1, M2 and substrate alignment marks P1, P2.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the support structure (e.g. mask table) MT and thesubstrate table WT are kept essentially stationary, while an entirepattern imparted to the radiation beam is projected onto a targetportion C at one time (i.e. a single static exposure). The substratetable WT is then shifted in the X and/or Y direction so that a differenttarget portion C can be exposed.

2. In scan mode, the support structure (e.g. mask table) MT and thesubstrate table WT are scanned synchronously while a pattern imparted tothe radiation beam is projected onto a target portion C (i.e. a singledynamic exposure). The velocity and direction of the substrate table WTrelative to the support structure (e.g. mask table) MT may be determinedby the (de-)magnification and image reversal characteristics of theprojection system PS.

3. In another mode, the support structure (e.g. mask table) MT is keptessentially stationary holding a programmable patterning device, and thesubstrate table WT is moved or scanned while a pattern imparted to theradiation beam is projected onto a target portion C. In this mode,generally a pulsed radiation source is employed and the programmablepatterning device is updated as required after each movement of thesubstrate table WT or in between successive radiation pulses during ascan. This mode of operation can be readily applied to masklesslithography that utilizes programmable patterning device, such as aprogrammable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

It will be understood that the lithographic apparatus is represented inFIG. 1 in a highly schematic form, but that is all that is necessary forthe present disclosure.

As shown in FIG. 2, the lithographic apparatus LA forms part of alithographic cell LC, also sometimes referred to a lithocell or cluster,which also includes apparatus to perform pre- and post-exposureprocesses on a substrate. Conventionally these include spin coaters SCto deposit resist layers, developers DE to develop exposed resist, chillplates CH and bake plates BK. A substrate handler, or robot, RO picks upsubstrates from input/output ports 1/01, 1/02, moves them between thedifferent process apparatus and delivers then to the loading bay LB ofthe lithographic apparatus. These devices, which are often collectivelyreferred to as the track, are under the control of a track control unitTCU which is itself controlled by the supervisory control system SCS,which also controls the lithographic apparatus via lithography controlunit LACU. Thus, the different apparatus can be operated to maximizethroughput and processing efficiency.

In order that the substrates that are exposed by the lithographicapparatus are exposed correctly and consistently, it is desirable toinspect exposed substrates to measure properties such as overlay errorsbetween subsequent layers, line thicknesses, critical dimensions (CD),etc. Accordingly, a manufacturing facility in which lithocell LC islocated also includes metrology system MET which receives some or all ofthe substrates W that have been processed in the lithocell. Metrologyresults are provided directly or indirectly to the supervisory controlsystem SCS. If errors are detected, adjustments may be made to exposuresof subsequent substrates, especially if the inspection can be done soonand fast enough that other substrates of the same batch are still to beexposed. Also, already exposed substrates may be stripped and reworkedto improve yield, or discarded, thereby avoiding performing furtherprocessing on substrates that are known to be faulty. In a case whereonly some target portions of a substrate are faulty, further exposurescan be performed only on those target portions which are good.

Within metrology system MET, an inspection apparatus is used todetermine the properties of the substrates, and in particular, how theproperties of different substrates or different layers of the samesubstrate vary from layer to layer. The inspection apparatus may beintegrated into the lithographic apparatus LA or the lithocell LC or maybe a stand-alone device. To enable most rapid measurements, it isdesirable that the inspection apparatus measure properties in theexposed resist layer immediately after the exposure. However, the latentimage in the resist has a very low contrast—there is only a very smalldifference in refractive index between the parts of the resist whichhave been exposed to radiation and those which have not—and not allinspection apparatuses have sufficient sensitivity to make usefulmeasurements of the latent image. Therefore, measurements may be takenafter the post-exposure bake step (PEB) which is customarily the firststep carried out on exposed substrates and increases the contrastbetween exposed and unexposed parts of the resist. At this stage, theimage in the resist may be referred to as semi-latent. It is alsopossible to make measurements of the developed resist image—at whichpoint either the exposed or unexposed parts of the resist have beenremoved—or after a pattern transfer step such as etching. The latterpossibility limits the possibilities for rework of faulty substrates butmay still provide useful information.

FIG. 3(a) shows schematically the key elements of an inspectionapparatus implementing so-called dark field imaging metrology. Theapparatus may be a stand-alone device or incorporated in either thelithographic apparatus LA, e.g., at the measurement station, or thelithographic cell LC. An optical axis, which has several branchesthroughout the apparatus, is represented by a dotted line O. A targetgrating structure T and diffracted rays are illustrated in more detailin FIG. 3(b).

As described in the prior applications cited in the introduction, thedark-field imaging apparatus of FIG. 3(a) may be part of a multi-purposeangle-resolved scatterometer that may be used instead of or in additionto a spectroscopic scatterometer. In this type of inspection apparatus,radiation emitted by a radiation source 11 is conditioned by anillumination system 12. For example, illumination system 12 may includea collimating lens system, a color filter, a polarizer and an aperturedevice 13. The conditioned radiation follows an illumination path IP, inwhich it is reflected by partially reflecting surface 15 and focusedinto a spot S on substrate W via a microscope objective lens 16. Ametrology target T may be formed on substrate W. Lens 16, has a highnumerical aperture (NA), preferably at least 0.9 and more preferably atleast 0.95. Immersion fluid can be used to obtain with numericalapertures over 1 if desired.

The objective lens 16 in this example serves also to collect radiationthat has been scattered by the target. Schematically, a collection pathCP is shown for this returning radiation. The multi-purposescatterometer may have two or more measurement branches in thecollection path. The illustrated example as a pupil imaging branchcomprising pupil imaging optical system 18 and pupil image sensor 19. Animaging branch is also shown, which will be described in more detailbelow. Additionally, further optical systems and branches will beincluded in a practical apparatus, for example to collect referenceradiation for intensity normalization, for coarse imaging of capturetargets, for focusing and so forth. Details of these can be found in theprior publications mentioned above.

Where a metrology target T is provided on substrate W, this may be a 1-Dgrating, which is printed such that after development, the bars areformed of solid resist lines. The target may be a 2-D grating, which isprinted such that after development, the grating is formed of solidresist pillars or vias in the resist. The bars, pillars or vias mayalternatively be etched into the substrate. Each of these gratings is anexample of a target structure whose properties may be investigated usingthe inspection apparatus.

The various components of illumination system 12 can be adjustable toimplement different metrology ‘recipes’ within the same apparatus. Inaddition to selecting wavelength (color) and polarization ascharacteristics of the illuminating radiation, illumination system 12can be adjusted to implement different illumination profiles. The planeof aperture device 13 is conjugate with a pupil plane of objective lens16 and the plane of the pupil image detector 19. Therefore, anillumination profile defined by aperture device 13 defines the angulardistribution of light incident on substrate W in spot S. To implementdifferent illumination profiles, an aperture device 13 can be providedin the illumination path. The aperture device may comprise differentapertures mounted on a movable slide or wheel. It may alternativelycomprise a programmable spatial light modulator. As a furtheralternative, optical fibers may be disposed at different location in theillumination pupil plane and used selectively to deliver light or notdeliver light at their respective locations. These variants are alldiscussed and exemplified in the documents cited above.

In a first example illumination mode, aperture 13N is used and rays 30 aare provided so that the angle of incidence is as shown at ‘I’ in FIG.3(b). The path of the zero order ray reflected by target T is labeled‘0’ (not to be confused with optical axis ‘0’). In a second illuminationmode, aperture 13S is used, so that rays 30 b can be provided, in whichcase the angles of incidence and reflection will be swapped comparedwith the first mode. In FIG. 3(a), the zero order rays of the first andsecond example illumination modes are labeled 0(13N) and 0(13S)respectively. Both of these illumination modes will be recognized asoff-axis illumination modes. Many different illumination modes,including on-axis illumination modes can be implemented for differentpurposes.

As shown in more detail in FIG. 3(b), target grating T as an example ofa target structure is placed with substrate W normal to the optical axisO of objective lens 16. In the case of an off-axis illumination profile,a ray of illumination I impinging on grating T from an angle off theaxis O gives rise to a zeroth order ray (solid line 0) and two firstorder rays (dot-chain line +1 and double dot-chain line −1). It shouldbe remembered that with an overfilled small target grating, these raysare just one of many parallel rays covering the area of the substrateincluding metrology target grating T and other features. Since the beamof illuminating rays 30 a has a finite width (necessary to admit auseful quantity of light), the incident rays I will in fact occupy arange of angles, and the diffracted rays 0 and +1/−1 will be spread outsomewhat. According to the point spread function of a small target, eachorder +1 and −1 will be further spread over a range of angles, not asingle ideal ray as shown.

In the branch of the collection path for dark-field imaging, imagingoptical system 20 forms an image T′ of the target on the substrate W onsensor 23 (e.g. a CCD or CMOS sensor). An aperture stop 21 is providedin a plane in the imaging branch of the collection path CP which isconjugate to a pupil plane of objective lens 16. Aperture stop 20 mayalso be called a pupil stop. Aperture stop 21 can take different forms,just as the illumination aperture can take different forms. The aperturestop 21, in combination with the effective aperture of lens 16,determines what portion of the scattered radiation is used to producethe image on sensor 23. Typically, aperture stop 21 functions to blockthe zeroth order diffracted beam so that the image of the target formedon sensor 23 is formed only from the first order beam(s). In an examplewhere both first order beams are combined to form an image, this wouldbe the so-called dark field image, equivalent to dark-field microscopy.As an example of an aperture stop 21, aperture 21 a can be used whichallows passage of on-axis radiation only. Using off-axis illumination incombination with aperture 21 a, only one of the first orders is imagedat a time.

The images captured by sensor 23 are output to image processor andcontroller PU, the function of which will depend on the particular typeof measurements being performed. For the present purpose, measurementsof asymmetry of the target structure are performed. Asymmetrymeasurements can be combined with knowledge of the target structures toobtain measurements of performance parameters of lithographic processused to form them. Performance parameters that can be measured in thisway include for example overlay, focus and dose. Special designs oftargets are provided to allow these measurements of differentperformance parameters to be made through the same basic asymmetrymeasurement method.

Referring again to FIG. 3(b) and the first example illumination modewith rays 30 a, +1 order diffracted rays from the target grating willenter the objective lens 16 and contribute to the image recorded atsensor 23. When the second illumination mode is used, rays 30 b areincident at an angle opposite to rays 30 b, and so the −1 orderdiffracted rays enter the objective and contribute to the image.Aperture stop 21 a blocks the zeroth order radiation when using off-axisillumination. As described in the prior publications, illumination modescan be defined with off-axis illumination in X and Y directions.

By comparing images of the target grating under these differentillumination modes, asymmetry measurements can be obtained.Alternatively, asymmetry measurements could be obtained by keeping thesame illumination mode, but rotating the target. While off-axisillumination is shown, on-axis illumination of the targets may insteadbe used and a modified, off-axis aperture 21 could be used to passsubstantially only one first order of diffracted light to the sensor. Ina further example, a pair of off-axis prisms 21 b are used incombination with an on-axis illumination mode. These prisms have theeffect of diverting the +1 and −1 orders to different locations onsensor 23 so that they can be detected and compared without the need fortwo sequential image capture steps. This technique, is disclosed in theabove-mentioned published patent application US2011102753A1, thecontents of which are hereby incorporated by reference. 2nd, 3rd andhigher order beams (not shown in FIG. 3) can be used in measurements,instead of or in addition to the first order beams. As a furthervariation, the off-axis illumination mode can be kept constant, whilethe target itself is rotated 180 degrees beneath objective lens 16 tocapture images using the opposite diffraction orders.

In the following disclosure, techniques will be illustrated formeasuring focus performance of a lithographic process that uses obliqueillumination on a reflective type of patterning device I assume we arenot excluding DUV scanners here? (transmissive reticles) At least weshouldn't. These techniques may be applied in particular in EUVlithography, where reflective optics in a near-vacuum environment arerequired. Metrology targets including certain focus metrology patternswill be printed on the substrate, at the same time as product featuresare printed. Asymmetry in these printed patterns will be measured usingfor example diffraction based techniques in the apparatus of FIG. 3. Toallow the use of small targets, it will be assumed that these asymmetrymeasurements will be performed using the dark-field imaging branch ofthe apparatus. Diffraction-based measurements of asymmetry can also bemade using the pupil imaging branch, however. Of course, the apparatusshown in FIG. 3 is only one example of an inspection apparatus andmethod that may be used to measure asymmetry.

In the context of lithographic apparatuses working in the DUV wavelengthrange, targets for diffraction-based focus (DBF) measurements have beendesigned and used successfully. A known type of DBF target is producedby including sub-segmented features in a grating pattern on the reticle.These features have dimensions below the imaging resolution of thelithographic apparatus, alongside more solid features. Consequently,they do not print as individual features in the resist layer on thesubstrate, but they influence the printing of the solid features, in amanner that is sensitive to focus error. Specifically, the presence ofthese features creates an asymmetric resist profile for each line in thegrating within the DBF metrology target, with the degree of asymmetrybeing dependent upon focus. Consequently a metrology tool such as theinspection apparatus of FIG. 3 can measure the degree of asymmetry froma target formed on the substrate, and translate this into the scannerfocus.

Unfortunately, the known DBF metrology target designs are not suitablefor use in all situations. In EUV lithography, resist film thicknessesare significantly lower than those used in DUV immersion lithography,leading to low diffraction efficiency and difficulty extracting accurateasymmetry information from diffracted radiation in the scatterometer. Inaddition, since the resolution of the imaging system is inherentlyhigher in EUV lithography, features having dimensions below the printingresolution of DUV immersion lithography become “solid” featuresprintable by EUV lithography. To provide analogous sub-resolutionfeatures on an EUV reticle is rather impractical, and/or may violatesemiconductor manufacturer's “design rules”. Such rules are generallyestablished as a means to restrict the feature designs to ensure theprinted features conform to their process requirements. In any case,working outside the design rules makes it difficult to simulate theperformance of the process on the DBF targets, so that the optimumtarget design and the calibration of focus measurements becomes a matterof trial-and-error. The desire to conform to design rules applies to DBFtargets in DUV lithography, not only EUV lithography.

The focus (DBF) metrology target should have a unique, and preferablymonotonic, asymmetry signal as a function of target defocus. In thiscontext, an asymmetry signal may describe a difference (e.g., anintensity and/or phase difference) in opposing higher diffraction orders(e.g., +1 and −1 diffraction orders). It is also important thatprecision and sensitivity is high. Other considerations includeminimizing dose and other cross talk effects (e.g., resultant fromprocessing effects), and the tool-to-tool matching between inspectiontools should be good.

It has been observed a good principle for a target is one based on adifferential of two Bossung like signals with a focus shift betweenthem. Most present techniques will yield a single Bossung, with no signinformation and zero sensitivity around best focus.

FIG. 4 illustrates a previous example for addressing the issues raised,as described in European Patent Application No. 17177774.1. The Figureshows in isolation a small portion of a focus metrology pattern. Therepeating unit of this pattern comprises one first feature 422 and onesecond feature 424, spaced from each other in the direction ofperiodicity. The direction of periodicity in this example may be the Xdirection of the patterning device and substrate. Each first feature 422comprises a bar or other feature each having a minimum dimension w1 thatis close to but not less than a resolution limit of the printing step.This value w1 may be for example less than 50 nanometers in thedirection of periodicity. A second space, between each first feature 422and its next nearest neighboring second feature 424, has a dimension w2′and is similar to the dimension w2 of the second features 424themselves. Consequently, it will be seen that the pattern T comprisingthin first features and thicker second features is effectively presentin both positive and negative form. Putting these dimensions w1, w1′together with the much larger period P, it will be appreciated that thedimensions w2 and w2′ are much greater than the minimum dimension w1 ofthe first features 422, and consequently much greater than theresolution limit of the printing step. Dimensions w2 and w2′ may eachbe, for example, over four times, over five, six, eight or ten times thedimension w1. Each second feature in the periodic array further includessub-features 426 having minimum dimensions close to but not less than aresolution limit of the printing step in a direction transverse to saiddirection of periodicity. The sub-features in this example are linesprojecting asymmetrically from a main body 428 of the first feature. Thelength of these projecting lines or fingers is labelled w3. The mainbody 428 of each second features 424 defines a minimum dimension w4 ofthe second features in the direction of periodicity. Accordingly, inthis notation, maximum dimension w2 of the second features 424 is equalto w3+w4. The minimum dimension of the sub-features in the transversedirection is labeled w5.

It is understood that it is only the gap between the sub-features 426and the first features 422 changes through focus. And moreover, thisvariation is linear or at least monotonous, resulting in a monotonousasymmetry signal when the target is measured. It is therefore assumedthat this gap depends on different Bossung behavior between that of afirst edge defined by the ends of the sub features and a second edgedefined by the first features 422. The first edge effectively pulls back(the length of sub-features 426 become smaller) with defocus while theposition of the second edge remains relatively stable. An effectiveBossung shift may also come from a physical height difference: thesub-features 426 being effectively sampled at a lower resist height thanthe first features 422.

However, the inventor believes that sensitivity to focus, with respectto the target design of FIG. 4, could be improved upon. Better focussensitivity means a stronger asymmetry signal and improved focusmeasurements.

FIG. 5 illustrates the principles of a method of measuring focusperformance of a lithographic apparatus according to the presentdisclosure. In the disclosed method, the lithographic apparatus is usedto print at least one focus metrology pattern T on a substrate W. Theprinted focus metrology pattern T comprises an array of features that isperiodic in at least one direction. For the purpose of this example, thefocus metrology pattern T is periodic in the Y direction, whichcorresponds to the scanning direction of the lithographic apparatus. Ina lithographic apparatus of the type described, the direction ofillumination is at an oblique angle, within the Y-Z plane. The focusmetrology pattern T is made periodic in the Y direction. By measuringasymmetry in the printed focus metrology pattern, for example using aninspection apparatus of the type described above, a measurement of focusperformance can be derived.

Patterning device MA comprises reflective and non-reflective portions todefine features of one or more device patterns and one or more metrologypatterns. As one type of metrology pattern of interest for the presentdisclosure, a focus metrology pattern T to be formed on the substrate Wis defined by a corresponding pattern T″ formed on reflective patterningdevice MA. An enlarged detail of part of the reticle is shown at 502.The printing operation which transfers this pattern onto a resist layeron substrate W is performed in the lithographic apparatus of FIG. 1 byilluminating the reticle with EUV radiation 504 radiation incident at anoblique angle θ, which may be for example in the range of 5° to 10°.Reflected radiation 506 carrying information of the metrology targetpattern (and all the product features desired to be printed on thesubstrate) enters the projection system PS. The basis of the reticle isa reflective structure 508, which is typically a multilayer structure,adapted to reflect a wavelength of radiation used in the lithographicapparatus. The EUV radiation is typically shorter than 20 nanometers.For example, a wavelength of approximately 13.5 nm is used in currentimplementations, which are based on a tin plasma radiation source.

On top of the reflective structure 508, radiation-absorbent structure510 is provided, which may comprise a layer of EUV-absorbing material,and optionally a protective capping layer. Structure 510 is selectivelyremoved so as to leave reflecting portions 512, with non-reflectingportions being defined by radiation-absorbent structure 514, inaccordance with the pattern that is desired to be printed ican pn theresist material on the substrate. Depending on the type of resistmaterial used, the developed pattern may have resist featurescorresponding to the reflective portions (negative tone resist) or tothe non-reflective portions (positive tone resist). For the presentillustration, a positive resist process will be assumed, unlessotherwise stated. The teaching of the present disclosure can readily beadapted by the skilled person to either type of process.

Focus metrology pattern T comprises a grating pattern with a length L ina direction of periodicity. The direction of periodicity in this exampleis the Y direction, as mentioned. The period P of the structure ismarked, and an enlarged portion of the pattern including one of therepeating units 600 is shown. Each repeating unit in this examplecomprises a periodic repetition of a pattern region 605 with a spaceregion 610. The skilled person will understand that the projectionsystem PS of a typical lithographic apparatus will apply a predeterminedde-magnification factor when printing the pattern from the patterningdevice MA onto the substrate W. Accordingly, the dimensions of featuresgiven in the following examples will be understood to refer to the sizesof features as printed on the substrate, and the sizes of thecorresponding features on the patterning device such as reticle 502 maybe physically several times larger. This scaling factor should be takenfor granted in the following description, and will not be mentionedagain. Similarly, unless the context otherwise requires, the dimensionsof features of the metrology pattern T are stated as would be if thepattern is perfectly transferred from the patterning device to theresist. As will be appreciated, the basis of the focus metrology methodis that the features will not be perfectly printed, when a non-zerofocus error is present.

The wavelength of radiation used in the printing step, for example EUVradiation, is much shorter than the wavelengths of radiation typicallyused to measure asymmetry in the inspection apparatus of FIG. 3. EUVradiation may be defined as radiation in the range 0.1 nm to 100 nmwhile the wavelength of radiation used in the printing step may be forexample less than 20 nanometers. The inspection apparatus in someembodiments may use visible or infrared radiation at one or morewavelengths in the range 200 to 2000 nm. The wavelength of radiationused in the printing step may be in such cases ten or more times shorterthan the wavelength of radiation used in the measuring of asymmetry. Inother examples, the wavelength of the measuring radiation may be shorterthan 200 nm, for example in the range 150-400 nm or even 100 nm to 200nm.

Whichever radiation wavelengths are used for the printing of the patternand the measuring of it, the focus metrology pattern contains featureswith a range of properties adapted to suit these conditions. Dimensionsof features comprised within the pattern region 605 are designed to havea dimension similar to the smallest features printed as part of theproduct patterns. If this were not so, focus sensitivity would besignificantly lower, resulting in lower precision. Furthermore, thisfocus performance measured using the focus metrology pattern T might notaccurately represent focus performance in the actual product features ofinterest.

On the other hand, in view of the longer wavelengths used in theinspection apparatus (even allowing for the fact that inspectionapparatus using shorter wavelengths might be applied), these individualfirst features are too small to be resolved directly by the inspectionapparatus. By arranging groups of first features in a grating patternhaving an overall period P that is comparable to the inspectionapparatus wavelength, a diffraction spectrum of the pattern as a wholebecomes resolvable in the inspection apparatus, and properties of thesmaller features can be inferred. The period P of the grating patternmay for example be 350 nm or 450 nm or 600 nm. The overall length L ofthe grating pattern may be, for example, 5 μm. Such a size allows thepattern to be included within the scribe lanes or even in the deviceareas, but still resolved using the dark-field imaging branch of theinspection apparatus of FIG. 3. (If measurements are to be made usingthe pupil imaging branch, then a larger target is typically required,for example with L of 30 μm or 40 μm so that the illumination spot S canbe placed entirely within the grating.) The relative sizes of featuresand gratings, and numbers of features in each grating are not intendedto be shown to scale in any of the figures here.

FIG. 6 shows various focus metrology patterns according to embodiments.It will be appreciated that these illustrations are purely exemplary,and of course other examples can be envisaged, based on the principlesdisclosed herein. In all of the examples, only a small section of thepattern is shown, including a repeating unit with period P.

FIG. 6(a) shows in isolation a small portion of the same focus metrologypattern that is used as the example in FIG. 5. This type of pattern maybe used to measure focus performance for an EUV lithographic processusing a reflective patterning device MA, or a conventional transmissiveprocess.

The target design comprises a periodic pattern 600 comprising repeatedinstances of a pattern region 605 and space region 610. The patternregion comprises at least one iteration of a first feature 615,hereafter referred to as a comb feature 615 and at least one secondfeature 620, hereafter referred to as a line feature 620. Comb feature615 comprises sub-features 625, which in this example are linesprojecting asymmetrically from a main body 630 of the first feature. Thelength of these projecting lines or fingers is labelled w7. In FIG.6(a), each pattern region 605 of periodic pattern 600 has one repetitionof comb feature 615 and line feature 620 (e.g., two of each), in FIG.6(b) each pattern region 605 of periodic pattern 600′ has only one combfeature 615 and one line feature 620, and in FIG. 6(c), each patternregion 605″ of periodic pattern 600 has two repetitions of comb feature615 and line feature 620. For each repetition (pair of first feature andsecond feature), the fingers 625 of the comb feature 615 are adjacent(i.e., facing) the at least one line feature 620. Note that this is incontrast to the FIG. 4 arrangement.

As previously discussed, a focus measurement response to the target,when formed on a substrate and measured, should comprise two shiftedBossung curves. In the proposed target design, a first Bossung responsemay result from the focus response of fingers 625 and a second Bossungresponse may result from the line feature 620. A relative shift in thepeaks of these first and second Bossung curves may result from the factthat the finger 625 is effectively sampled at a lower height(z-direction) in resist. An additional asymmetry mechanism may be basedon the sidewall angle of the fingers 625.

It is believed that an about 1:1 line-space ratio defined by (i.e., theratio of) the pattern region 605 and space region 610 may yield amaximum signal (asymmetry) intensity. It is this line-space periodicpitch that is detected by the metrology device (scatterometer) during ameasurement; the sub-features 625 and main body 630 of comb feature 615are effectively seen as a thick line by a scatterometer. It is alsobelieved that repeating the near resolution or at-resolution featureswill multiply the area which provides the focus-dependent signal, andthus the signal. The example of FIG. 6(a) has an effective line-spaceratio of about 1:1 (i.e., w1 is about equal to w2), and one repetitionof near resolution features. Therefore, this may be the betterarrangement in terms of signal strength. However, it is also expectedthat the examples of FIGS. 6(b) and (c) will provide acceptableperformance. For example, the width of the pattern region 605 and spaceregion 610 may differ by no more than 100%, by no more than 50%, by nomore than 20% or by no more than 10%. Alternatively. or in addition, interms of line-space ratio (ratio between the pattern region 605 andspace region 610), the ratio of the largest of these to the smallest ofthese (i.e., the largest:smallest ratio) should be less than 3:1, lessthan 2:1, or less than 3:2, for example.

Several parameters of the focus metrology pattern can be adjusted aspart of a design process for an optimal focus metrology pattern. Theoptimal focus metrology may be different for each layer and each processof a product, particularly where operating parameters of thelithographic apparatus may be customized for each layer. Dimensions w1,w2, w3, w4, w5, w6, w7, w8, w9, pitch P and the finger pitch Pt of combfeature 615 may all vary, (between targets and/or even within a target).For example, pitch P may be between 300 nm and 800 nm or between 300 nmand 600 nm; for example period or pitch P may be 350 nm, 450 nm or 600nm. Design parameters may be expressed in any suitable format. Ratiossuch as the ones just given may be convenient for expressing relativedimensions of features, while absolute dimensions may be expresseddirectly, or by ratios relative to a specified resolution limit, and/orrelative to the period P. In the illustrated example, with a period P of450 or 600 nm, the linewidths w3, w4 and w8 (for, respectively, the mainbody of comb features, the line features and the fingers) and the spacesw5, w6, w9 (spaces between the comb features and the line features, andbetween each finger) may all have the same dimension; e.g., to be on theorder of between 20 nm and 40 nm, e.g., 22 or 30 nm. The dimension w7(length of the fingers 625) may be approximately twice the linewidthe.g., to be on the order of between 40 nm and 80 nm, e.g., 44 or 60 nm.

Generally speaking, the person skilled in imaging technology willconsider that features are effectively isolated from one another, if thespace between them is five or six times dimensions of the featuresthemselves. Here, the maximum spacing between each of comb feature andeach line feature within each pattern region is not greater than twicethe resolution limit of the lithography apparatus used to print thetarget. For example, this spacing may be no greater than 100 nm, nogreater than 80 nm, no greater than 60 nm, no greater than 40 nm, nogreater than 30 nm or no greater than 20 nm. Thus, in this example, thefeatures 615, 620 of each pattern region 605 are not isolated from eachother, but each pattern region 605 is isolated from its neighboringpattern region(s).

FIG. 7 shows a number of variations on the targets described herein. Inthe FIG. 6 embodiments, the (thin) line feature 620 is effectivelyfloating which is not ideal. In terms of imaging, the last line in asimple grating always prints differently to the other lines because itdoes not have a neighboring structure. For a good signal, the last thinline should print the same as all other thin lines. Therefore, it isproposed that the last thin line is thickened or comprises an additionaldummy neighbor, to maximize this similarity. In FIG. 7, the effectivethickness of the right edge of each pattern region 705 has beenincreased to improve stability of this edge. FIGS. 7(a), (c) and (e) areexamples with no repetition, respectively, of equivalent singlerepetition examples of FIGS. 7(b), (d) and (f). More specifically, thepattern illustrated in FIGS. 7(a), (c) and (e) comprises a patternregion 705 having a single comb feature 715 and line feature 735, 735′,735″ alternating with a space region 710, and the pattern illustrated inFIGS. 7(b), (d) and (f) comprises a pattern region 705 having onerepetition of comb feature 715 and line feature 720, 735, 735′, 735″alternating with a space region 710. The basic principles can beextended to other repetition examples, e.g., the two repetition exampleof FIG. 6(c), where space allows. In the illustrated embodiment, it canbe seen that for the multiple repetition examples, the pattern does notrepeat exactly; i.e., right hand line feature 735, 735′, 735″ isdifferent to other line features 720.

In FIGS. 7(a) and (b) the right hand line feature 735 has been madethicker. For example it may be about 1.5 times or twice as thick as theline feature 720 (e.g., about 1.5 times or twice as thick as the linefeature 620 of FIG. 6). Note that, for the single repetition example ofFIG. 7(b), it is only the second line feature 735 which is made thicker.In FIGS. 7(c) and (d), the right hand line feature 635′ comprises a dualthin-thin line, e.g., two lines, each being similar to that of linefeature 720. In FIGS. 7(e) and (f), the right hand line feature 635″comprises a dual thin-thick line, e.g., two lines, the outermost linebeing thicker than the innermost line. For example the thinner line maybe similar to that of line feature 720 and the thicker line may be 1.5times or twice as thick.

While targets including the above focus metrology target patterns mayyield focus measurements (when appropriately designed for the process),there is also an expectation that the focus measurement of a target willbe subject to uncertainty because of the wide variety of aberrationsand/or image distortions that can be introduced, besides focus.Accordingly, embodiments of the measurement method are also disclosed inwhich multiple differential measurements are made on two or more focusmetrology patterns. These may be provided in complementary pairs, withmirrored asymmetry in their designs, and/or in pairs with designdifferences other than mirror symmetry.

FIG. 8 illustrates two complementary focus metrology patterns that canbe used together to obtain an improved measurement of focus. Purely byway of example, the pattern of FIG. 6(a) has been selected as the basisfor this complementary pair, as seen in FIG. 8(a). The other pattern ofthe pair seen at FIG. 8(b) is a mirror image.

FIG. 9 shows the printing of two or more complementary patterns side byside on a substrate W, forming a composite focus metrology target T. Inthis particular example, there are four focus metrology patterns,arranged in two complementary pairs TNa/TMa and TNb/TMb. In eachcomplementary pair, the first pattern (printed on the right) is labeledTN (using N for ‘normal’) while the second pattern is printed on theleft and labeled TM (M for ‘mirror’). It will be understood that thelabels are arbitrary, but the effect is that the printed focus metrologypattern comprises at least first and second periodic arrays of features,each periodic array of features forming an individual focus metrologypattern. There is then a programmed asymmetry within each periodicarray, the asymmetry of the second periodic array being opposite to thatof the first periodic array, to form a complementary pair. To obtain animproved focus measurement then includes measuring asymmetry of each ofthe first and second periodic arrays and determining a measure of focusperformance by combining the asymmetries measured for the periodicarrays (TN, TM).

By combining results from measurements using targets that have oppositeasymmetries in their designs, the focus measurement can be made lesssensitive to asymmetries that arise in the projection system or themetrology system, that otherwise might be mistaken for focus error.Particular types of imperfection that can be discriminated using acomplementary pair of patterns in this way are coma and projectionasymmetry. For example, coma may be expected to introduce asymmetry in aparticular direction, when the image is defocused. By contrast, theasymmetry induced by focus error will be opposite in the “mirrored”pattern compared with the “normal” pattern. Combining the asymmetrymeasurements from both allows the actual focus error to be moreaccurately measured.

Additionally, in this example, two complementary pairs of targets areprovided, identified by the suffixes ‘a’ and ‘b’. Between these pairs,the design parameters of the focus metrology patterns are varied. As afirst difference, the period Pa of the pair TNa/TMa is longer than theperiod Pb of the pair TNb/TMb, and lengths of the “fingers” have beenshortened. In other embodiments, different parameters could be varied(e.g., any one or more of w1, w2, w3, w4, w5, w6, w7, w8, w9 or Pt), andthe periods P could be the same or different. Alternatively or inaddition to providing different pattern designs, different captureconditions can also be used to obtain more diverse signals. For example,different wavelengths and/or polarizations of radiation can be used toobtain diffraction signals.

As illustrated in FIG. 9, therefore, a composite focus metrology targetT can be formed by one or more complementary pairs focus metrologypatterns TN and TM being printed in the same step. As illustrated, theseindividual patterns may be imaged simultaneously using radiation spot Sin the dark field imaging mode of the inspection apparatus of FIG. 3. Inother words, measurements of asymmetry in both of these focus metrologypatterns can be taken by taking first and second images using the +1 and−1 order diffracted radiation collected by the apparatus. One such imageis shown in FIG. 10. The dark rectangle represents the dark-field imageas recorded on sensor 23 in the apparatus of FIG. 3, for example. Acircle S′ indicates the area of radiation spot S, imaged onto thedetector. Brighter rectangles TNa′, TNb′, TMa′ and TMb′ represent theimages of the corresponding focus metrology patterns TNa, TNb, TMa andTMb, respectively. The intensity of one diffraction order from eachtarget can be measured by, for example, defining a region of interestROI within each of the brighter rectangles, and averaging the pixelvalues. Repeating this for the opposite diffraction order allowsasymmetry to be calculated. In an alternative measurement method usingthe prisms 21 b shown in FIG. 3, then effectively both images of bothpatterns can be captured simultaneously.

The principles illustrated in FIGS. 8 to 10 can be applied to any of thepatterns illustrated in FIGS. 6 and 7.

In yet other embodiments, asymmetry of each focus metrology pattern maybe measured separately, for example using the pupil imaging branch ofthe inspection apparatus of FIG. 3, or a more general angle-resolvedscatterometer. The opposite diffraction orders from one pattern arelocated in complementary regions of the pupil image, but only onepattern can be measured at a time.

FIG. 11 is a flowchart of the steps of a method for measuring focusperformance of a lithographic process according to an exemplaryembodiment. The method can be performed using any of the example focusmetrology patterns described above and illustrated in the drawings. Thesteps are as follows, and are then described in greater detailthereafter:

1000—Start by defining a product design or metrology wafer design withmetrology targets, and preparing a suitable set of patterning devices(reticles). In advance of production, make exposures with knownfocus-exposure variations and measure these to obtain one or morecalibration curves. (This may involve an iterative loop of design,exposure and measurement steps.)

1010—Print one or more focus metrology patterns alongside productpatterns on a substrate;

1020—Measure intensity of a portion of the diffraction spectrum of eachfocus metrology pattern using a suitable inspection apparatus (forexample the +1 order is a suitable portion of the diffraction spectrum);

1030—Measure intensity of an opposite portion of the diffractionspectrum (for example, −1 order) of each focus metrology pattern usingthe inspection apparatus;

1040—Calculate measurements of asymmetry of one or more focus metrologypatterns by comparing the intensities of the opposite diffractionorders;

1050—Using the asymmetry measurements, with the calibration curvesstored in step 1000 and/or other measurements such as SEM, calculatefocus error at the time of printing the focus metrology pattern.

1060—Use the derived focus measurement in focus setting for exposures onsub sequent substrates.

1070—End or repeat.

As already explained, step 1020 and step 1030 may be performed as asingle step such that the opposite diffraction orders of a focusmetrology pattern can be obtained in a single acquisition. In addition,where there are two or more patterns being measured, for example one ormore complementary pairs of patterns shown in FIG. 9, oppositediffraction orders for these two or more patterns may be measured usinga single image acquisition, to obtain a corresponding number ofasymmetry measurement values.

Although the measurement steps are shown being made by a scatterometer,as a dedicated inspection apparatus, this may be a stand-alone apparatusor it may be integrated in the lithocell. Moreover, asymmetrymeasurements can be made without dedicated metrology apparatus, forexample using suitable targets with the alignment sensors provided inthe lithographic apparatus.

Calculation steps 1040 and 1050 can all be performed in a processor ofthe inspection apparatus, or may be performed in different processorsassociated with monitoring and control of the lithographic apparatus.Each step may be performed by a programmed processor, and it is anadvantage of the techniques disclosed, that the inspection apparatus canbe modified to perform the focus measurement methods without hardwaremodification.

Further embodiments are disclosed in the subsequent numbered clauses:

-   -   1. A method of measuring focus performance of a lithographic        apparatus, the method comprising:        -   (a) obtaining measurement data relating to measured            asymmetry between opposite portions of a diffraction            spectrum for a first periodic array in a printed focus            metrology pattern on a substrate; and        -   (b) deriving a measurement of focus performance based at            least in part on the asymmetry comprised within the            measurement data,        -   wherein said first periodic array comprises a repeating            arrangement of a space region having no features and a            pattern region having at least one first feature comprising            sub-features projecting from a main body and at least one            second feature; and wherein the first feature and second            feature are in sufficient proximity to be effectively            detected as a single feature when measured in a measurement            step.    -   2. A method as defined in clause 1, wherein a maximum spacing        between each of said at least one first feature and at least one        second feature within each pattern region is not greater than        twice the resolution limit of the printing step.    -   3. A method as defined in clause 1 or 2, wherein a maximum        spacing between each of said at least one first feature and at        least one second feature within each pattern region is not        greater than 80 nm.    -   4. A method as defined in any preceding clause, wherein said        each said sub-features project asymmetrically from the main        body.    -   5. A method as defined in any preceding clause, wherein each        second feature is located on the same side of the main body of a        respective one of said first features as from which said        sub-features extend.    -   6. A method as defined in any preceding clause, wherein an        outermost second feature of each pattern region has an effective        thickness at least twice that of the spacing between first        feature and second feature and/or the main body of said first        feature.    -   7. A method as defined in clause 6, wherein said outermost        second feature comprises either:    -   a single line feature having a thickness of at least 1.5× that        of the spacing between first feature and second feature and/or        the main body of said first feature,    -   a dual line feature comprising two adjacent lines, or    -   a dual line feature comprising two adjacent lines wherein the        outermost line has a width at least 1.5 times that of the        spacing between first feature and second feature and/or the main        body of said first feature.    -   8. A method as defined in any preceding clause, wherein each        second feature comprises a line.    -   9. A method as defined in any preceding clause, wherein each        pattern region comprises at least one repetition of said first        feature and second feature.    -   10. A method as defined in any preceding clause, wherein a        minimum dimension of each first feature and second feature is        close to but not less than a resolution limit of the printing        step.    -   11. A method as defined in any preceding clause, wherein the        largest: smallest ratio of the width of the space region and the        width of the pattern region in the direction of periodicity is        less than 3:1.    -   12. A method as defined in any preceding clause, wherein the        largest:smallest ratio of the width of the space region and the        width of the pattern region in the direction of periodicity is        less than 2:1.    -   13. A method as defined in any preceding clause, wherein the        largest:smallest ratio of the width of the space region and the        width of the pattern region in the direction of periodicity is        less than 3:2.    -   14. A method as defined in any preceding clause wherein the        printed focus metrology pattern comprises at least first and        second periodic arrays of features, each periodic array of        features having a form as specified in said preceding clause,        wherein there is a programmed asymmetry within each periodic        array, the asymmetry of the second periodic array being opposite        to that of the first periodic array, and wherein said asymmetry        within the measurement data comprises asymmetry of each of the        first and second periodic arrays and step (b) determines said        measure of focus performance by combining the asymmetries        correspond to the periodic arrays.    -   15. A method as defined in clause 14 wherein said sub-features        are arranged such that each second feature is asymmetric within        regard to the direction of periodicity, and wherein the        asymmetry of each second feature in the second periodic array of        features is opposite to that in the first periodic array of        features.    -   16. A method as defined in any preceding clause wherein the        measurement in step (a) is performed using radiation having a        wavelength at least twice as long as said minimum dimension of        the first features and/or second features.    -   17. A method as defined in any preceding clause, wherein the        measurement data corresponds to a measurement performed using        radiation having a wavelength longer than 150 nm while a        wavelength of radiation used in printing the said focus        metrology pattern is less than 20 nm.    -   18. A method as defined in any preceding clause, wherein the        period of each of said periodic arrays of features in said focus        metrology pattern is greater than 350 nm.    -   19. A method as defined in any preceding clause, comprising        using inspection radiation to measure said asymmetry between        opposite portions of a diffraction spectrum for the first        periodic array in the printed focus metrology pattern.    -   20. A method as defined in any preceding clause, comprising        using the lithographic apparatus to print at the least one focus        metrology pattern on the substrate, the printed focus metrology        pattern comprising at least said first periodic array of        features.    -   21. A patterning device for use in a lithographic apparatus, the        patterning device comprising reflective and non-reflective        portions to define features of one or more device patterns and        one or more metrology patterns, the metrology patterns including        at least one focus metrology pattern, the focus metrology        pattern comprising at least a first periodic array of features        comprising a repeating arrangement of features arranged to        define a space region having no features and a pattern region        having at least one first feature comprising sub-features        projecting from a main body and at least one second feature; and        wherein the first feature and second feature are in sufficient        proximity to be effectively detected as a single feature during        a scatterometery based metrology action to measure asymmetry        between opposite portions of a diffraction spectrum for the        first periodic array as formed on a substrate.    -   22. A patterning device as defined in clause 21, wherein a        maximum spacing between each of said at least one first feature        and at least one second feature within each pattern region are        such that imaged features formed on a substrate corresponding to        each of said at least one first feature and at least one second        feature when using said patterning device in an imaging step is        not greater than 80 nm, taking into account any magnification        factor applicable to the imaging step.    -   23. A patterning device as defined in clause 21 or 22, said        sub-features project asymmetrically from the main body.    -   24. A patterning device as defined in clause 23, wherein each        second feature is located on the same side of the main body of a        respective one of said first features as from which said        sub-features extend.    -   25. A patterning device as defined in clause 23 or 24, wherein        an outermost second feature of each pattern region has an        effective thickness at least twice that of the spacing between        first feature and second feature and/or the main body of said        first feature.    -   26. A patterning device as defined in clause 25, wherein said        outermost second feature comprises either:    -   a single line feature having a thickness of at least 1.5 times        that of the spacing between first feature and second feature        and/or the main body of said first feature,    -   a dual line feature comprising two adjacent lines, or    -   a dual line feature comprising two adjacent lines wherein the        outermost line has a width at least 1.5 times that of the        spacing between first feature and second feature and/or the main        body of said first feature.    -   27. A patterning device as defined in any of clauses 21 to 26,        wherein each second feature comprises a line.    -   28. A patterning device as defined in any of clauses 21 to 27,        wherein each pattern region comprises at least one repetition of        said first feature and second feature.    -   29. A patterning device as defined in any of clauses 21 to 28,        wherein the largest:smallest ratio of the width of the space        region and the width of the pattern region in the direction of        periodicity is less than 3:1.    -   30. A patterning device as defined in any of clauses 21 to 28,        wherein the largest:smallest ratio of the width of the space        region and the width of the pattern region in the direction of        periodicity is less than 2:1.    -   31. A patterning device as defined in any of clauses 21 to 28,        wherein the largest:smallest ratio of the width of the space        region and the width of the pattern region in the direction of        periodicity is less than 3:2.    -   32. A patterning device as defined in in any of clauses 21 to        31, wherein the focus metrology pattern comprises at least first        and second periodic arrays of features, said repeating        arrangement of features, wherein there is a programmed asymmetry        within each periodic array, the asymmetry of the second periodic        array being opposite to that of the first periodic array.    -   33. A metrology apparatus for measuring a parameter of a        lithographic process, the metrology apparatus being operable to        perform the method of any of clauses 1 to 20.    -   34. A lithographic system comprising:    -   a lithographic apparatus comprising:    -   an illumination optical system arranged to illuminate a        reflective patterning device;    -   a projection optical system arranged to project an image of the        patterning device onto a substrate; and    -   a metrology apparatus according to clause 33;    -   wherein the lithographic apparatus is arranged to use the        measurement of focus performance derived by the metrology        apparatus when applying the pattern to further substrates.    -   35. A lithographic system as defined in clause 34, wherein said        reflective patterning device comprises the patterning device of        any of clauses 21 to 32.    -   36. A lithographic cell comprising the metrology apparatus        according to clause 33 or the lithographic system according to        clause 34 or 35.    -   37. A computer program comprising processor readable        instructions which, when run on suitable processor controlled        apparatus, cause the processor controlled apparatus to perform        the method of any of clauses 1 to 20.    -   38. A method of manufacturing devices wherein a device pattern        is applied to a series of substrates using a lithographic        process, the method including:    -   using the method of any of clauses 1 to 20 to measure focus        performance of the lithographic process, and    -   controlling the lithographic process for later substrates in        accordance with the measured focus performance.

CONCLUSION

In conclusion, a method of manufacturing devices using the lithographicprocess can be improved by performing focus measurement methods asdisclosed herein, using it to measure processed substrates to measureparameters of performance of the lithographic process, and adjustingparameters of the process (particularly focus) to improve or maintainperformance of the lithographic process for the processing of subsequentsubstrates.

While the target structures including and focus metrology patternsdescribed above are metrology targets specifically designed and formedfor the purposes of measurement, in other embodiments, properties may bemeasured on targets which are functional parts of devices formed on thesubstrate. Many devices have regular, grating-like structures. The terms“metrology pattern” and “metrology target” and the like as used hereindo not require that the structure has been provided specifically for themeasurement being performed.

The substrates on which these metrology patterns are formed may beproduction wafers or experimental wafers in product development. Theymay also be dedicated metrology wafers, for example monitor wafers whichare processed intermittently as part of an advance process control (APC)mechanism.

In association with the physical grating structures defining the focusmetrology patterns as realized on substrates and patterning devices, anembodiment may include a computer program containing one or moresequences of machine-readable instructions describing a method ofdesigning focus metrology patterns, metrology recipes and/or controllingthe inspection apparatus to implement the illumination modes and otheraspects of those metrology recipes. This computer program may beexecuted for example in a separate computer system employed for thedesign/control process. As mentioned, calculations and control steps maybe wholly or partly performed within unit PU in the apparatus of FIG. 3,and/or the control unit LACU of FIG. 2. There may also be provided adata storage medium (e.g., semiconductor memory, magnetic or opticaldisk) having such a computer program stored therein.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g., having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

1.-15. (canceled)
 16. A method of measuring focus performance of alithographic apparatus, the method comprising: obtaining measurementdata relating to measured asymmetry between opposite portions of adiffraction spectrum for a first periodic array in a printed focusmetrology pattern on a substrate; and deriving a measurement of focusperformance based at least in part on the asymmetry comprised within themeasurement data, wherein the first periodic array comprises a repeatingarrangement of a space region having no features and a pattern regionhaving at least one first feature comprising sub-features projectingfrom a main body and at least one second feature, and wherein the firstfeature and second feature are in proximity to one another so as to bedetected as a single feature when measured in a measurement step. 17.The method of claim 16, wherein a maximum spacing between each of the atleast one first feature and the at least one second feature within eachpattern region is at least one of: not greater than twice the resolutionlimit of the printing step; and not greater than 80 nm.
 18. The methodof claim 16, wherein the sub-features project asymmetrically from themain body.
 19. The method of claim 16, wherein each second feature islocated on the same side of the main body of a respective one of thefirst features as from which the sub-features extend.
 20. The method ofclaim 16, wherein an outermost second feature of each pattern region hasan effective thickness at least twice that of the spacing between firstfeature and second feature and/or the main body of the first feature,and wherein the outermost second feature comprises either: a single linefeature having a thickness of at least 1.5× that of the spacing betweenfirst feature and second feature and/or the main body of the firstfeature, a dual line feature comprising two adjacent lines, or a dualline feature comprising two adjacent lines wherein the outermost linehas a width at least 1.5 times that of the spacing between first featureand second feature and/or the main body of the first feature.
 21. Themethod of claim 16, wherein at least one of: each second featurecomprises a line; each pattern region comprises at least one repetitionof the first feature and second feature; a minimum dimension of eachfirst feature and second feature is close to but not less than aresolution limit of the printing step; and the largest-to-smallest ratioof the width of the space region and the width of the pattern region inthe direction of periodicity is less than 3:1 or less than 2:1, or lessthan 3:2.
 22. The method of claim 16, wherein the printed focusmetrology pattern comprises at least first and second periodic arrays offeatures, wherein there is a programmed asymmetry within each periodicarray, the asymmetry of the second periodic array being opposite to thatof the first periodic array, and wherein the asymmetry within themeasurement data comprises asymmetry of each of the first and secondperiodic arrays and the deriving determines the measure of focusperformance by combining the asymmetries correspond to the periodicarrays, and wherein the sub-features are arranged such that each secondfeature is asymmetric within regard to the direction of periodicity, andwherein the asymmetry of each second feature in the second periodicarray of features is opposite to that in the first periodic array offeatures.
 23. The method of claim 16, wherein the measurement isperformed using radiation having a wavelength at least twice as long asthe minimum dimension of the first features and/or second features. 24.The method of claim 16, comprising using inspection radiation to measurethe asymmetry between opposite portions of a diffraction spectrum forthe first periodic array in the printed focus metrology pattern.
 25. Apatterning device for use in a lithographic apparatus, the patterningdevice comprising: reflective and non-reflective portions that definefeatures of one or more device patterns and one or more metrologypatterns, wherein the metrology patterns include at least one focusmetrology pattern comprising at least a first periodic array of featureshaving a repeating arrangement of features arranged to define a spaceregion having no features and a pattern region having at least one firstfeature comprising sub-features projecting from a main body and at leastone second feature, and wherein the first feature and the second featureare in proximity to be detected as a single feature during ascatterometery-based metrology action to measure asymmetry betweenopposite portions of a diffraction spectrum for the first periodic arrayas formed on a substrate.
 26. A metrology apparatus for measuring aparameter of a lithographic process, the metrology apparatus beingoperable to perform the method of measuring focus performance of alithographic apparatus, the method comprising: obtaining measurementdata relating to measured asymmetry between opposite portions of adiffraction spectrum for a first periodic array in a printed focusmetrology pattern on a substrate; and deriving a measurement of focusperformance based at least in part on the asymmetry comprised within themeasurement data, wherein the first periodic array comprises a repeatingarrangement of a space region having no features and a pattern regionhaving at least one first feature comprising sub-features projectingfrom a main body and at least one second feature; and wherein the firstfeature and second feature are in proximity to one another so as to bedetected as a single feature when measured in a measurement step.
 27. Alithographic system comprising: a lithographic apparatus comprising: anillumination optical system arranged to illuminate a reflectivepatterning device; a projection optical system arranged to project animage of the patterning device onto a substrate; and a metrologyapparatus of claim 26; wherein the lithographic apparatus is arranged touse the measurement of focus performance derived by the metrologyapparatus when applying the pattern to further substrates.
 28. Alithographic cell comprising the metrology apparatus of claim
 26. 29. Acomputer program comprising processor readable instructions which, whenrun on suitable processor controlled apparatus, cause the processorcontrolled apparatus to perform the method of measuring focusperformance of a lithographic apparatus, the method comprising:obtaining measurement data relating to measured asymmetry betweenopposite portions of a diffraction spectrum for a first periodic arrayin a printed focus metrology pattern on a substrate; and deriving ameasurement of focus performance based at least in part on the asymmetrycomprised within the measurement data, wherein the first periodic arraycomprises a repeating arrangement of a space region having no featuresand a pattern region having at least one first feature comprisingsub-features projecting from a main body and at least one secondfeature; and wherein the first feature and second feature are inproximity to one another so as to be detected as a single feature whenmeasured in a measurement step.
 30. A method of manufacturing deviceswherein a device pattern is applied to a series of substrates using alithographic process, the method including: using the method of claim16, to measure focus performance of the lithographic process, andcontrolling the lithographic process for later substrates in accordancewith the measured focus performance.
 31. A lithographic cell comprisingthe lithographic system of claim 27.